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Generator sets vibrate. A seemingly obviousstatement, but one with considerable implica-tions in relation to interconnections betweenthe generator set and surrounding structure.Interconnections between a generator setand a building, housing, oundation or otherstructure must be exible and able to absorbthe vibration movement o the unit. I they arenot, they can ail, causing damage, down time

or the generator and potential dangers topersonnel. Flexible interconnections includemechanical connections such as uel lines,exhaust pipes and air ducts. They also includeelectrical connections or power, control,auxiliaries, etc. The purpose o this technicaltopic is to review and highlight the require-ments and importance o exible connectionsto generator sets with specifc attention to the

AC power connections.

In general, designers o installations or, andinstallers o, generator sets seem cognizanto the need or exible interconnection pointsbetween the generator set and surroundingstructures. In practice these interconnectionswould seem to be designed and installedadequately. A lack o reported problems orailures in this area supports these assertions.It has recently come to the attention oCummins Power Generation that occasionally

AC power connection schemes have not pos-sessed adequate exibility or the applications.These conditions are primarily notable wheregenerator sets are at higher voltages, i.e. 5 kVand 15 kV, where the cables become quite sti,or in cases where conduits are believed to beexible but actually provide little exibility.

Understanding the issue

Oten the engine/alternator combination is mountedto a skid rail using vibration isolators integral to thedesign. Alternatively, and sometimes in additionto these isolators, spring type isolators are placedbetween the chassis and the oundation. When thereis a sub base uel tank, the isolators are oten placedbetween the skid and tank. Any o these and othervariations are designed to accomplish the same tasko isolating vibrations rom the structure or oundationand preventing the orces rom damaging the generatorset equipment. In doing so they all allow movement othe engine/alternator and anything that is attached to it,with respect to the oundation. These movements can

be large and visible to the eye or minimal and higher inrequency. In any case, a solid connection between theengine/alternator and the surrounding structure will besubject to the orces o this movement and may break,bend or atigue over time. I the inadequate connec-tions themselves do not ail, it is possible the orceswill be transmitted to generator set components andthe condition will maniest itsel as a ailure o one othem. Even a generator set that is solidly mounted andwithout isolators can vibrate enough to require exibleinterconnections.

> White paper

By Timothy A. Loehlein, Technical Specialist-Electrical

Flexible interconnectionsor generator set AC powercables and conduits

Power topic #6011 | Technical inormation rom Cummins Power Generation

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2006 Cummins Power Generation

next o the same design. Also the installation designmay vary considerably. Unique considerations such asspecialized equipment or seismic zone installation mayalso need to be considered. A good rule o thumb is toinclude at least one bend in the conduits and cables.The addition o one bend will add the third dimension

o movement capability.Solutions

As mentioned, one solution to assure the ability toabsorb in three dimensions is to include at least one

bend in exible conduitand/or cables, withinbend limits o the conduitand cable, o course,between the generatorset connection enclosureand the frst clampingpoint at the oundation

or structure (FIGURE 2).With the addition o onlyone bend, three dimen-sional movementis allowed.

A potential solution is to utilize cable trays or wireways,instead o conduit buried in concrete. FIGURE 3 is anexample where this type o installation allows verticalmovement along with the horizontal exibility o thesheathed cable itsel. Note that in this case, in essencea single bend has been included as the cable leavesthe wireway. This bend plus the wireway construction,

allows or movement in any dimension. This sameapproach could apply or bottom cable entry with awireway beneath. A conduit with a bend in a concreteoor will not provide or this movement because thebend is restricted by the concrete and conduit.

Another possible alterna-tive is to utilize a cableo increased exibility.This type o cable is notallowed in conduit andso requires an additional

junction point between

the cables leaving theconduits and the genera-tor connection box. Anexample o such an ar-rangement is illustrated inFIGURE 4 where exible

Power topic #6011 | Page 2

One o the keys to under-standing the subtletieso achieving adequateexibility is to realize thatthe vibration movementis three dimensional.

Engine fring pulses androtation o crankshatand generator rotor,even when balanced,create movement directlyor indirectly in threedimensions. This meansthere is three dimensional

movement between generator set components such asthe output box and the oundation or structure. Sometypes o cables, conduits or installation arrangementwill absorb movement in two dimensions but not three.FIGURE 1 illustrates this in reerence to conduits

standing vertically. In the two horizontal dimensionsthe conduit will move and absorb vibration. Verticallyhowever, vibration movement is in a direction thatattempts to compress or stretch along the length othe conduits. This direction o movement oten cannotbe absorbed. In this case there is a perception o goodvibration isolation but this is not correct, resulting inpotential ailure.

In FIGURE 1, note that the conduit is liquid tight conduit,generally considered somewhat exible. The picturedinstallation imposes the compression/stretching orcesmentioned earlier, which this conduit cannot absorb.

In this case the sti connection between the outputbox and the concrete translates into movement andstress upon the sheet metal box, contributing to metalatigue. Due to the stiness o the HV cables, they alsoexperience the same stresses, and i the conduits weredecoupled the cables would experience higher stresslevels. These stresses can be transmitted to the cableconnection bushings and could lead to serious ailure.

Startup or shutdown o a generator set oten producesmovement that is more exaggerated than the vibra-tion during continuous operation. In this case theamount o movement (dimension) can be more criticalthan the requency or short duration. So the exibleconnections must have enough movement capabilityto absorb start and stopping conditions as well asrunning conditions. To try to put a defned specifcationon this is impossible rom a practical standpoint. Eachunit will vary rom other designs and rom one to the

FIGURE 1

FIGURE 3

FIGURE 2

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leads and bellows absorbvibration movement.(Conduits and associatedwiring is not shown).

Summary

Flexible cable connec-tions to generator sets arerequired to allow or threedimensional vibrationmovement between thegenerator set and theoundation or surroundingstructure where cablesand conduits are secured.Lack o adequateexibility can result indamage or ailure o thecables, conduits and/or

terminations; or can result in excessive orces beingtranserred to other components o the generator set orto the surrounding structure. The presence o at leastone bend in exible conduits or cables between the

generator set and the last securing point to surroundingstructure will allow or three dimensional movement.

Assurance o adequate exible characteristics omaterials used is also key. Always ollow electricalcodes and other codes applicable to the installation.

Design Engineer, Technical Project Leader and ProjectManager. His current position is a Technical Specialist-Electrical in application engineering or CumminsEnergy Solutions Business, supporting combined heatand power (CHP) and peaking applications.

About the author

Timothy A. Loehlein is a graduate o theUniversity o Minnesota with a Bacheloro Electrical Engineering and a PE inMinnesota. Tim has been a CumminsPower Generation employee since 1976in positions as Application Engineer,

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2007 Cummins Power Generation Inc. All rights reserved.

Cummins Power Generation and Cummins are registered

trademarks o Cummins Inc. Our energy working or you.

is a trademark o Cummins Power Generation.

PT-6011 (07/07) Formerly 900-0296

FIGURE 4

L0

L1

L2

L3

Customer Cables Enter at Bottom

(No Flex Required)

Flexible

Cables to

Generator

For additional technical support, please contact yourlocal Cummins Power Generation distributor. To locateyour distributor, visit www.cumminspower.com.